1. Field of the Invention
The invention relates to an amplifier, and more particularly, to an amplifier with a common-mode feedback (CMFB) circuit.
2. Description of the Related Art
Regarding a fully differential operational amplifier (FD Op-Amp) with a feedback circuit, the feedback circuit determines only differential output voltages, but does not affect common-mode output voltages. Thus, an additional circuit is required for the FD Op-Amp to control the common-mode output voltage Vcmo, so that the common-mode output voltage Vcmo approaches a reference voltage value (usually halfway between two operating voltages). Referring to FIG. 1, a FD Op-Amp with a common-mode feedback circuit 100 comprises a FD Op-Amp 110, a common-mode detector 120 and a CMFB amplifier 130. Both the common-mode detector 120 and the CMFB amplifier 130 form a common-mode feedback circuit. The configuration and the operation of the common-mode feedback circuit are described on pages 816-835 of a book entitled “Analysis and Design of Analog Integrated Circuits,” by Gray et al, 4th Edition, 2001, Wiley, and on pages 314-324 of a book entitled “Design of Analog CMOS Integrated Circuits,” by Razavi, 2001, McGraw Hill.
The FD Op-Amp with a CMFB circuit 100 needs to be well compensated; otherwise, a noise injection into the common-mode output voltage Vcmo could cause the common-mode output voltage Vcmo to ring or oscillate. Usually, the compensation method for amplifier 100 can be classified as source degeneration and current reduction. The source degeneration compensation is to provide a resistor coupled between two transistors at two input terminals of the CMFB amplifier 130, enhancing stability by reducing the gain of the CMFB amplifier 130. The current reduction compensation is to reduce the amount of the control current of the FD Op-Amp 110 to 1/N (N is an integer and N>>1) so as to enhance circuit stability. FIG. 2 shows two frequency responses, one is an uncompensated FD Op-Amp with a CMFB circuit and the other is a compensated FD Op-Amp with a CMFB circuit using above-mentioned compensation method. Referring to FIG. 2, it is obvious that the compensated FD Op-Amp using above-mentioned compensation method sacrifices larger gain and larger bandwidth in order to obtain stability.